(1) Field of the Invention
The present invention is directed to the production of hard, rubber, sport play balls, and particularly to one-and two-piece homogeneous golf balls, as well as to curable compositions from which such articles of manufacture are produced. More particularly, the present invention relates to the production of golf balls comprising a major proportion of cis-1,4- polybutadiene elastomer crosslinked with a coagent comprising (i) an admixture of a polyvalent metal salt of an unsaturated acid and an active hydrogen-containing organic filler, or (ii) a reaction product obtained by the reaction of an unsaturated carboxylic acid with an active hydrogen containing organic filler, followed by further reaction with a polyvalent metal compound in the presence of said unsaturated carboxylic acid whereby said coagent serves as a crosslinking agent with the polybutadiene elastomer. The incorporation of the coagent results in a golf ball exhibiting improved compression and fracture strength while maintaining desired rebound, click and feel required in a golf ball, as compared with golf balls produced from cis-1,4-polybutadiene elastomers devoid of the incorporation of the coagent of the invention.
(2) Description of the Prior Art
The technical evolution of the contemporary solid elastomer two-piece golf ball, as opposed to the ballata covered rubber thread wound balls (three-piece balls) has progressed to the stage where the average golfer may achieve satisfactory play with the two-piece homogeneous solid ball, but as a result of lack of durability, dependability and feel, i.e. click, of the ball, the professional or more astute golfer cannot rely upon the performance of the state of the art two-piece ball for serious or tournament play. The one-piece ball, although much described in the literature, is in reality not a practical item of commerce, simply because the compositions employed in formulation of the one-piece ball provide no greater degree of impact durability than do the compositions described to be respresentative of a core formulation for two-piece balls. The latter, however, circumvent the lack of impact resistance by encasing the core in a safety shell of high impact resistant polymers of various descriptions, most frequently described as "ionomer copolymers" manufactured and sold by E. I. Dupont de Nemours & Co. Inc., as "Surlyn.TM.". In general, the prior art, illustratively, U.S. Pat. Nos. 3,313,545; 3,438,933; 4,056,269; and 4,169,599, disclose the production of homogeneous golf balls by molding a composition comprising an elastomer, a crosslinking monomer and filler. According to these patent disclosures, the filler is distributed throughout the structure and serves a variety of functions, for example, improving the tensile and compression strength as well as adjusting the density to the USGA specified value for golf balls.
Golf balls manufactured as one-piece or two-piece balls based on the aforedescribed prior art compositions all develop a common problem. Upon aging, i.e. storage following their manufacture, these compositions continue to harden and consequently embrittle. After the composition has achieved a sufficiently high state of embrittlement upon impaction, as would be received from the head of the golf club, a fissure is developed, which upon a second or subsequent impact, results in one or more splits in the core or ball. When the two-piece ball is involved, the cracked aggregates in the encasement will shift on further impacts, resulting in the displacement of the center of gravity of a flying golf ball, and thus causing its flight to be totally unpredictable. The one-piece ball scenario is much simpler; if a fissure results, subsequent club hit impacts will cause the ball to crack into at least two pieces, thereby either destroying the ball or rendering the same unsuitable for continued play.
Hence, while such balls as described in the prior art have been generally satisfactory, it is an object of the present invention to improve the properties of such molded homogeneous balls still further with respect to compression and fracture strength, particularly upon aging, while maintaining desired rebound, flight characteristics, distance, cut and chip resistance, click and feel, required in a golf ball.